Wire bonding is used to attach fine lead wires, typically 25 μm to 75 μm (i.e., 1–3 mils) in diameter, from one bond pad to another to complete an electrical connection in electronic devices. Lead wires are frequently made of gold, aluminum, silver, or copper. The bond pads can be interconnection areas formed on a semiconductor chip or metallized areas on interconnection substrates. In plastic encapsulated devices, semiconductor dice are wire bonded to metal lead frames. According to recent literature, approximately 4×1012 lead wires are bonded every year, mostly in producing 40 to 50 billion integrated circuits fabricated annually, worldwide. Contemporary methods of wire bonding include wedge bonding and ball bonding. Both methods utilize thermocompression, ultrasonic, and thermosonic techniques. All of these techniques are well-known in the art and all rely on good mechanical and electrical contact between the lead wire and the bond pad.
FIG. 1A shows a plan view of a typical ball-bonded interconnect area 100. The bonded interconnect area 100 includes a bond pad 101, a lead wire 103, and a wire interconnect portion 105. The wire interconnect portion 105 is a portion of the lead wire 103 deformed through a combination of heat and ultrasonic energy applied while connecting the lead wire 103 to the bond pad 101. The bond pad 101 is frequently square in shape with a typical dimension, s, being 70 μm to 100 μm on a side and is typically located on a periphery of a silicon die for making connection with one of a plurality of package pins (not shown). A diameter, d, of the lead wire may be from 25 μm to 75 μm; as described above. FIG. 1B shows an isometric view of the bonded interconnect area 100. The bonded interconnect area 100 in FIG. 1B more clearly illustrates the wire interconnect portion 105 and additionally indicates a surface interface 107 between the wire interconnect portion 105 and the bond pad 101.
A majority of contemporary IC wafers fabricated employ aluminum-copper (or aluminum-copper-silicon) metallization for wire bond pads. A large proportion of all semiconductor-device failures are caused by wires inadequately bonded to a bond pad and known failure mechanisms are limited. A predominant failure mechanism is galvanic corrosion occurring on a bond pad. Galvanic corrosion occurs during a wafer saw operation to singulate the dice in a wafer. The formation of galvanic corrosion on the aluminum alloy bond pads causes voids, particles (e.g., Al2Cu particles), and a thin layer of aluminum hydroxide (Al(OH)3) on a topmost surface of the bond pads (see, for example, “Micro-Corrosion of Al—Cu Bonding Pads”; S. Thomas et al., IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-10, No. 2, June 1987, pgs. 252–257). The Al2Cu particles act as a local microscopic cathode in contact with the more anodic aluminum, causing corrosion in the presence of an electrolyte. The electrolyte here is deionized water used during a dicing operation. Even “high-resistivity unrecirculated deionized (DI) water” can cause the pitting problem (Thomas et al., pg. 256). Researchers have attempted to dry the bond pads after sawing but have had limited success. Further, there is no easy way to scale the drying operation to large scale manufacturing environments. Therefore, the galvanic corrosion problem continues to plague manufacturing lines.
FIG. 2 shows a cross-section 200 of the bond pad 101 of FIGS. 1A and 1B. A topmost surface of the bond pad 101 is shown exhibiting galvanic corrosion 201 and Al2Cu particles 203. Studies, such as that of Thomas et al., supra, indicate the Al2Cu particles are typically about 1 μm in “diameter.” The galvanically corroded surface 201 and the Al2Cu particles 203 are the prime failure mechanisms preventing good mechanical and electrical bonding from occurring between the interface 107 of a wire interconnect portion 105 of the lead wire 103 and the bond pad 101 (FIGS. 1A and 1B).
Therefore, what is needed is a robust manufacturing method that economically and effectively controls the formation of galvanic corrosion on the bond pad 101.